Entry of Ebolavirus (EBOV) into host cells is mediated by its sole glycoprotein, known as GP. The GP and its associated EBOV entry events possess many unusual features that provide novel insights into our fundamental understanding of viral entry. In this R21 project, we aim to elucidate how the newly identified cellular restricton factors, known as interferon-inducible transmembrane (IFITM) proteins, especially IFITM2, potently and specifically inhibit EBOV entry, and in doing so, aid the development of novel antiviral therapeutics. Aim 1: Establish a single virus fusion assay for EBOV and dissect the stages of membrane fusion inhibited by IFITM2. We will take advantage of the fact that EBOV GP can be efficiently incorporated into its virus-like particles (VLPs) formed by the VP40 matrix protein, and develop a single virus imaging and fusion system to determine how IFITM2 inhibits EBOV fusion in endolysosomes. Aim 2: Elucidate the molecular and biochemical mechanisms by which IFITM2 specifically inhibits EBOV GP-mediated entry. We will test the central hypothesis that IFITM2 profoundly inhibits EBOV entry by disturbing the triggering capability and/or the cholesterol transport activity of its intracellular receptor, Niemann-Pick C1 (NPC1). A series of biochemical and novel fluorescence lipid labeling techniques will be used to assess the effect of IFITM2 on cholesterol content, membrane fluidity, and conformational changes of EBOV GP. EBOV is a highly pathogenic filovirus that causes severe hemorrhagic fever in humans, with a fatality rate of up to 90%. Results from the proposed studies will provide critical novel insight into how IFITM2 restricts EBOV GP-mediated membrane fusion and entry, as well as advance our understanding of the general mechanism of IFITMs that block viral entry.